Carbon fibers are used in a variety of composite materials because of their excellent characteristics such as high strength, high elastic modulus, and high conductivity. In addition, carbon fibers exhibit excellent mechanical strengths. Due to their conductivity, carbon fibers or carbon materials can be utilized in a variety of fields. In recent years, in conjunction with developments in electronic techniques, carbon fibers have been regarded as a promising filler in conducting resins for producing electromagnetic shielding materials or antistatic materials. Also, with the trend that resins have come to be used in the manufacture of automobiles in order to reduce their weight, carbon fibers have been seen as a useful antistatic filler that can be incorporated into the resins employed in automobiles.
Conventional carbon fibers, i.e., organic carbon fibers, are produced on a large scale by subjecting organic fibers, such as PAN-, pitch-, or cellulose-based fibers, to heat treatment and carbonization. In general, when carbon fibers are used as a filler in fiber-reinforced composite materials, in order to increase the contact area between the carbon fiber and the matrix of the material, the diameter of the fiber is reduced or the length thereof is increased. As a result, the reinforcement effect on the composite material is enhanced. In order to improve adhesion between the carbon fiber and the matrix, the carbon fiber preferably has a rough surface rather than a smooth surface. Therefore, the carbon fiber is subjected to surface treatment. For example, the carbon fiber is oxidized by exposure to air at a high temperature, or a coating agent is applied onto the surface of the fiber.
However, conventionally, it has been impossible to produce fine carbon fibers since the filament of organic fiber as the raw material has a diameter of at least about 5 to 10 μm. Furthermore, the ratio of length to diameter (i.e., aspect ratio=length/diameter) of conventional carbon fiber is limited. Because of these limitations, there has been a keen demand for the development of carbon fibers of a small diameter and a high aspect ratio.
When carbon fibers are incorporated into resins used for producing an automobile body, or in resins or rubbers for producing an electronic device, the carbon fibers must have conductivity comparable to that of a metal. Therefore, carbon fibers serving as a filler material have been required to have an improved conductivity.
In order to improve conductivity, carbon fibers must be subjected to graphitization to thereby increase the degree of crystallinity. Therefore, carbon fibers are usually subjected to graphitization at a high temperature. However, even when carbon fibers are subjected to such a graphitization, they still fail to attain conductivity comparable to that of a metal. Therefore, when a composite material is produced by use of a carbon fiber, in order to compensate for a low conductivity of carbon fiber itself, a large amount of carbon fiber is to be incorporated into the composite material. However, in this case, the workability and mechanical characteristics of the composite material are impaired. Therefore, in view of practical use, it is necessary to make further improvements to the conductivity of carbon fiber. In addition, it is also necessary to enhance the strength of the carbon fiber by reducing its diameter.
In the late 1980's, a vapor grown carbon fiber was produced through a process that differed from that used for producing a carbon fiber through carbonization and graphitization from an organic fiber such as PAN.
The vapor grown carbon fiber (hereinafter abbreviated as “VGCF”) is produced through thermal decomposition of hydrocarbon gas in a vapor phase in the presence of a metallic catalyst. Through this process, a carbon fiber having a diameter of hundreds of nm to 1 μm can be produced.
A variety of processes for producing VGCF are known, including a process in which an organic compound such as benzene, serving as a raw material, and an organic transition metal compound such as ferrocene, serving as a metallic catalyst, are introduced into a high-temperature reaction furnace together with a carrier gas, to thereby produce VGCF on a substrate (Japanese Patent Application Laid-Open (kokai) No. 60-27700); a process in which VGCF is produced in a free state (Japanese Patent Application Laid-Open (kokai) No. 60-54998); and a process in which VGCF is grown on a reaction furnace wall (Japanese Patent Application Laid-Open (kokai) No. 7-150419).
Through the aforementioned processes, there can be produced a carbon fiber of a relatively small diameter and a high aspect ratio that exhibits excellent conductivity and is suitable as a filler material. Therefore, a carbon fiber having a diameter of about 100 to 200 nm and an aspect ratio of about 10 to 500 is mass-produced, and is used, for example, as a conducting filler material in conducting resins or as an additive in lead storage batteries.
A characteristic feature of a VGCF filament resides in its shape and crystal structure. A VGCF filament has a structure including a very thin hollow part in its center portion, and a plurality of carbon hexagonal network layers whose crystals surround the hollow part like annual rings.
However, conventionally, VGCF having a small diameter of less than 100 nm cannot be produced on a large scale.
Recently, Iijima, et al. have discovered a multi-layer carbon nano-tube, which is a type of carbon fiber having a diameter smaller than that of VGCF, derived from soot obtained by evaporating a carbon electrode through arc discharge in helium gas. The multi-layer carbon nano-tube has a diameter of 1 to 30 nm, and is a fine carbon fiber filament having a structure similar to that of a VGCF filament. That is, the tube has a structure including a hollow part in its center portion, and a plurality of carbon hexagonal network layers whose crystals are superimposed in the form of annular rings around the hollow part.
However, the above process for producing the nano-tube through arc discharge is not carried out in practice, since the process is not suitable for mass production.
Meanwhile, a carbon fiber having a high aspect ratio and exhibiting a high conductivity could possibly be produced through the vapor-growth process, and therefore attempts have been made to improve the vapor-growth process for the production of carbon fiber of a smaller diameter. For example, U.S. Pat. No. 4,663,230 and Japanese Patent Examined Publication (kokoku) No. 3-64606 disclose a graphitic cylindrical carbon fibril having a diameter of about 3.5 to 70 nm and an aspect ratio of 100 or more. The carbon fibril has a structure in which a plurality of layers of ordered carbon atoms are continuously disposed concentrically about the cylindrical axis of the fibril, and the C-axis of each of the layers is substantially perpendicular to the cylindrical axis. The entirety of the fibril includes no thermally decomposed carbon overcoat deposited through thermal decomposition, and has a smooth surface.
Japanese Patent Application Laid-Open (kokai) No. 61-70014 discloses a carbon fiber having a diameter of 10 to 500 nm and an aspect ratio of 2 to 30,000, which fiber is produced through a vapor-growth process. According to this publication, a carbon layer obtained through thermal decomposition has a thickness of 20% or less of the diameter of the carbon fiber.
The both carbon fibers described above have smooth fiber surfaces. They exert substantially no frictional force because their surfaces are less uneven and they exhibit poor chemical reactivity since they have smooth fiber surfaces. Therefore, when such a carbon fiber is used in a composite material, the fiber must be subjected to surface treatment; for example, the surface of the fiber must be subjected to a satisfactory degree of oxidation.